1. Field of the Invention
The present invention relates to a vibrator to be used for a vibrating gyroscope, and a vibrating gyroscope using the vibrator. More particularly, the present invention relates to a vibrator which is capable of detecting rotational angular velocities around two, non-parallel axes, and which is used for hand shake prevention of a video camera, the navigation system of an automobile, a pointing device or the like, and a vibrating gyroscope using the vibrator.
2. Description of the Related Art
A vibrating gyroscope which is generally known as a conventional vibrating gyroscope and has a tuning-fork type vibrator or a sound-piece type vibrator can detect only one rotational angular velocity around one axis. In recent years, however, the market has demanded a vibrating gyroscope capable of detecting two rotational angular velocities around two axes to provide increased sensitivity and accuracy.
To satisfy the demand, the applicant of the present invention has already proposed two types of vibrating gyroscopes as discussed below.
FIG. 8 shows a first example of a vibrating gyroscope disclosed in Japanese Unexamined Patent Publication No. 7-19878.
A vibrating gyroscope 100 includes a first riangular prismatic vibrator 101 and a second triangular prismatic vibrator 102. The first vibrator 101 is provided with a first vibrating body 103 and two first piezoelectric elements 105 formed on two side surfaces of the first vibrating body 103. Only one of the two first piezoelectric elements 105 is shown in the figure. The second vibrator 102 is provided with a second vibrating body 104 and two second piezoelectric elements 106 formed on two side surfaces of the second vibrating body 104. only one of the two second piezoelectric elements 106 is shown in the figure.
The first vibrator 101 is joined with the second vibrator 102 so that the first vibrator 101 is approximately orthogonal to the second vibrator 102 in the vicinity of a center part of a surface on which no piezoelectric element is formed in the first vibrating body 103 and the second vibrating body 104, respectively.
In the vibrating gyroscope 100 of such a construction, the two first piezoelectric elements 105 and the two second piezoelectric elements 106 are connected to an output end of a drive circuit (not shown). Further, the two first piezoelectric elements 105 are connected to an input end of a first detection circuit (not shown). Still further, the two second piezoelectric elements 106 are connected to an input end of a second detection circuit (not shown).
In this vibrating gyroscope 100, a similar drive signal is inputted from the drive circuit to the two first piezoelectric elements 105 and the two second piezoelectric elements 106, and the first vibrator 101 and the second vibrator 102 are respectively flexural-vibrated in a direction orthogonal to the joining surface of the first vibrating body 103 with the second vibrating body 104.
When a rotational angular velocity around the axis of the first vibrator 101 is added in this condition, a Coriolis force is generated in a direction orthogonal to the vibrating direction. The vibrating direction of the first vibrator 101 is changed by the Coriolis force and results in a signal according to the rotational angular velocity being generated between the two first piezoelectric elements 105. The signal is detected by a detection circuit to output a detected signal corresponding to the rotational angular velocity around the axis of the first vibrator.
Similarly, when the rotational angular velocity around the axis of the second vibrator 102 is added, the signal according to the rotational angular velocity is generated between the two second piezoelectric elements 106. This signal is detected by the detection circuit to output a detected signal corresponding to the rotational angular velocity around the axis of the second vibrator.
This vibrating gyroscope 100 is thus capable of detecting two rotational angular velocities around two axes, i.e., the rotational angular velocity around the axis of the first vibrator 101 and the rotational angular velocity around the axis of the second vibrator 102.
The vibrating gyroscope 100 has the following problems. First, an interference (a beat) is generated when the resonance frequencies of two vibrators are close to each other. A false angular velocity signal can, thus, be generated from each vibrator. This necessitates sufficiently separating the resonance frequencies in the driving direction and the detecting direction of the respective vibrators from each other by taking countermeasures such as by shaping each vibrator differently and completely attenuating the beat frequency whose component includes the difference in the resonance frequencies by using, e.g., a low-pass filter.
Respective drive circuits and detection circuits are required for the two kinds of vibrators, resulting in a number of circuits twice that of a single axis gyroscope, thereby doubling the cost.
A beat can be eliminated by exciting vibrators in two directions at the same frequency and the same phase. That is, a beat can be eliminated if the two resonance frequencies of the two vibrators in the driving direction are in complete agreement with each other.
However, it is practically impossible to make two resonance frequencies completely agree with each other, taking into consideration the temperature characteristic.
FIG. 9 shows a second example of a vibrating gyroscope disclosed in Japanese Unexamined Patent Publication No. 6-3153.
The vibrating gyroscope 110 includes a disk-like vibrating body 112. Fan-shaped piezoelectric elements 114a, 114b, 114c, 114d, 114e, 114f, 114g, 114h whose central angle is approximately 45.degree. are formed on one major plane of the vibrating body 112, and these piezoelectric elements 114a-114h are used for detection to obtain a signal corresponding to the rotational angular velocity.
A disk-shaped piezoelectric element 116 is formed on the other major plane of the vibrating body 112, and used for driving in order to flexural-vibrate the vibrating body 112.
The piezoelectric elements 114a,114d and the piezoelectric elements 114e,114h are connected to a first detection circuit (not shown), while the piezoelectric elements 114b,114g and the piezoelectric elements 114c,114f are connected to a second detection circuit (not shown).
Here, the x-axis direction is defined as the direction orthogonal to the major plane of the vibrating body 112; the y-axis direction is defined as the direction which is orthogonal to the x-axis direction and passes between the piezoelectric elements 114a, 114b, 114g, 114h and the piezoelectric elements 114c, 114d, 114e, 114h, 114f; and the z-axis direction is defined as the direction which is orthogonal to the x-axis direction and passes between the piezoelectric elements 114a, 114b, 114c, 114d and the piezoelectric elements 114e, 114f, 114g, 114h.
When a drive signal from a drive circuit (not shown) is applied to the piezoelectric element 116, the vibrating body 112 is vibrated in the so-called concentric mode wherein a center part of the disk is vibrated reciprocally in the x-axis direction.
When a rotational angular velocity is applied around the z-axis in this condition, a Coriolis force is exerted in the y-axis direction. A difference is generated thereby between the voltage generated in the piezoelectric elements 114a,114d and the voltage generated in the piezoelectric elements 114e,114h, and the difference is detected by the first detection circuit to obtain the rotational angular velocity applied around the z-axis.
Similarly, when a rotational angular velocity is applied around the y-axis, a Coriolis force is exerted in the z-axis direction. A difference is generated thereby between the voltage generated in the piezoelectric elements 114b,114g and the voltage generated in the piezoelectric elements 114c, 114f, and the difference is detected by the second detection circuit to obtain the rotational angular velocity applied around the y-axis.
Thus, the vibrating gyroscope 110 is capable of detecting two rotational angular velocities around two axes, i.e., the rotational angular velocity around the y-axis and the rotational angular velocity around the z10 axis in the vibrating body 112.
In the vibrating gyroscope 110, the interference of two resonance frequencies around two axes which is the problem raised in the first conventional example is solved because only one vibrating body is present and is vibrated in a mode where the resonance frequencies in the driving direction and in the detecting direction are in complete agreement with each other around two axes.
Also, only one drive circuit is sufficient, and the cost of the circuit is reduced compared with that of the first conventional example.
The second conventional example is of a plane structure comprising flat plates, which is excellent in workability compared with a structure with a three-dimensional arrangement as in the first conventional example.
However, in the second conventional example, the change in the flexural displacement of the piezoelectric element by the Coriolis force generated in the horizontal direction of a flat-shaped vibrating body is detected. Generally speaking, when the force in the horizontal direction is applied to the flat plate, the deformation of the flat plate caused by the force is very small, and even when the force is applied to the flat plate vibrating body, the displacement of the flat plate is small, and thus, the flexural displacement of the piezoelectric element is also small, as is the detected signal to be obtained. Accordingly, only a vibrating gyroscope of low sensitivity can be obtained.